The present invention generally relates to the fabrication of power converter circuitry formed on substrates which are made from low temperature co-fired ceramic material and which have the passive components as an integral part of the substrate. Previously, power converter substrates were formed on circuit boards, or as power hybrids. Printed circuit boards provide traces for interconnecting a number of discrete components which are usually soldered to the circuit board. Component mounting may be either surface or through hole. Hybrids are created using thick and/or thin film manufacturing techniques to create a single surface substrate which usually has non-critical, low power dissipation resistive elements integrally formed with it. Some small value, low power inductive elements can be formed on the surface of the hybrid with thick film techniques but most magnetic components must be discrete, and mounted external to the hybrid structure (usually attached to the power converter heat sink). As with the inductive components, low voltage capacitors can be formed on the top layer of the substrate using thick film processes. However, these techniques are not suitable for the construction of high value, power or current handling components. These are realized as discrete devices and are mounted either on the surface of the hybrid (for the smaller parts) or more frequently external to the hybrid. Active components, such as semiconductor devices, are typically epoxied to the surface of the hybrid substrate, and wire bonded using gold or aluminum wire to bonding sites on the surface of the hybrid substrate.
Both printed circuit boards and hybrids have disadvantages in fabrication. Printed circuit boards require the use of numerous discrete components. Each must be inserted into or onto the circuit board, where leads may require shaping or trimming, and then sent through a solder bath to attach the components to the circuit board. This can expose the components and the circuit board to temperatures in excess of 600 degrees Fahrenheit. Active components are especially sensitive to elevated temperatures and may be damaged by heating. Especially with high frequency applications, small precision inductors and capacitors are easily damaged by being exposed to this high temperature. In the case of surface mounted components, where the component leads may require bending and trimming, and the part properly oriented and placed on the surface, this problem of soldering heat exposure is also encountered. The preceding considerations apply both to wave soldering and hand soldering.
Although the criticisms of the state of the art listed in the previous paragraph can be substantiated as generally accurate, lest they give the mistaken impression that current methods are quite inadequate, it must be added that many good power supplies have been built with those techniques and used with satisfaction by their operators. For example, exposure of components to high temperature during soldering can unquestionably damage the devices, but this can be and often is avoided by proper techniques and controls in the manufacturing process along with good engineering practice in the design of the components. The present invention is superior in that it generally eliminates or greatly reduces the exposure of components to the stress of solder attachments (though this advantageous feature is not advocated as the most significant feature of the presently disclosed invention).
This invention can be constructed and frequently will be constructed with some soldered components (the active components, certain very large value energy storage devices, or certain very high precision components) which either can not be integrated into the low temperature co-fired ceramic substrate, or are used in discrete form at the discretion of the designer. However, this invention will always have a greatly reduced number of solder connections compared to present techniques. This will translate into a statistical reduction in the number of defective solder joints and thereby improve overall manufacturing yield.
The reduced number of solder connections in this invention is a major benefit to the long term reliability of power supplies, which typically have significant internal power dissipation. This can subject the components and connections to large and frequent temperature changes in addition to those caused by variations in the ambient temperature of the unit operating environment. This results in the application of significant mechanical stress on the solder joints and can lead to increasing failures as the product ages, particularly those with latent defects; however, the present invention ameliorates and alleviates this well known problem.
Variations in components, solder composition, heating of the solder, placement of the small components on the circuit board, variations in hole plating, and components floating partially out of the holes in the printed circuit board all lead to variations between devices manufactured on the same production line. This means that two consecutive power conversion devices manufactured on printed circuit boards can have a variation in characteristics significant enough to cause additional circuitry to compensate for the variability to be incorporated into the design, or expensive material and process controls added to the manufacturing process. In either case, the result is a significant increase in the cost of the power supply.
This matter of variability control is an important advantageous characteristic of the present invention which is significant not only in the assembly procedures which it eliminates, but with the materials, processes and components themselves. In the present art, each individual discrete component is made with a variety of different materials whose composition will vary from manufacturing lot to lot. Each is also fabricated with a variety of different processes, each element and step of which has its own variability. This variability is ultimately reflected in the characteristics and cost of the discrete components. The discrete components can even be made by different manufacturers who use totally different materials, processes, and controls. The only present, effective methods of limiting this variability are through the use of tight specifications which can place severe material and process controls on the component manufacturer, and compliance testing and screening to verify the effectiveness of the process controls. Both of these can significantly increase the cost of the discrete components and therefore the entire power supply.
The above-described variability is also not fixed or even defined with time. It is common for component manufacturers to "improve" their products and processes. Sometimes the use of these improved devices (the older forms of which are no longer made) results in power supplies, which have been produced and worked well for years, suddenly developing a variety of problems or not functioning at all (either generally or in particular applications or under particular conditions). Therefore the advantageous feature of variability control is expected to enhance the commercial utility of the present invention.
Hybrid fabrication also has its characteristic difficulties. Thin film hybrid techniques generally result in a superior conductive layer with respect to thick film techniques because a uniform thickness layer of gold or other conductive material is etched away to leave a uniform thickness conductor which has a precise predetermined width. Resistive inks are then screened onto the hybrid substrate and trimmed to the proper value by laser trimming, or sandblasting techniques (although there are precision thin film resistors which do not require trimming). Capacitors cannot be integrally combined with a thin film hybrid substrate.
Capacitors are attached by the use of solder paste, conductive epoxy, and/or wire bonding. Conductive epoxy is not as conductive. Also, solder preforms may be used. Active components are wire bonded to conductive pads on the hybrid substrate. Wire bonding techniques are not perfect, and wire bonding is usually followed by a wire bond pull test to insure the integrity of the wire bonds. Additionally, the wire bond must be made directly to the bonding pad of the semiconductor device. This requires local ultrasonic heating and vibration to etch the wire bond material into the wire bond site of the semiconductor device. Each pad must be individually bonded to the substrate. The thickness of each wire that can be bonded to the semiconductor device and the substrate is limited. Techniques have been applied that use multiple wire bonds or ribbon bonding to increase the current carrying capability of the connection. The use of these techniques increases the current carrying capability, but increases the number of fabrication steps required, and the complexity of those fabrication steps. Wire bonds are flexible and may be of varying lengths, thereby changing minute characteristics of inductance, etc.
There is a further limitation on the usefulness of thick film hybrid technology. It is well known that following firing, thick film resistors do not remain fixed during subsequent firings but continue to change characteristics. The subsequent firings appear to a previously fired resistor as an extended firing period with included temperature cycling under a different set of conditions (covered by the fired and/or dried material of subsequently applied layers). At best, the resistor tolerance is a function of the number of substrate firing cycles subsequent to its own formation and the tolerances associated with this processing. If they otherwise could be used as precision resistors, it would be necessary to locate them as close to the top of the substrate as possible (probably on a single layer), limit the number of resistive paste materials (probably to one), and severely restrict the total number of precision resistors (and networks) in the circuit. Furthermore, due to the limited thickness of hybrid film resistors, their power dissipation capability is small which limits their usefulness for power supply applications. As a result, thick film resistors are best utilized in low power dissipation circuit applications in which values are not critical.
Additionally, fabrication of printed circuit boards requires the extensive use of acids, photoresist, and other chemicals to deposit and etch circuit boards. Thin film hybrid manufacturing similarly requires use of numerous acids and photoresist to etch away metallization. Thick film hybrid manufacturing does not use the wide variety of active chemicals that are used in the manufacture of thin film hybrids, and printed circuit boards. However, because of the manner in which conductive layers are screened onto a thick film hybrid, the resistivity, shape and dimensional stability of a thick film hybrid circuit is more difficult to control.
The present invention solves the problems associated with building power conversion circuitry in hybrids or on standard printed circuit boards by reducing the number of manufacturing steps, reducing the variety of manufacturing steps, and implementing manufacturing steps and processes that can be more accurately controlled and with less effort than previous technology.
Also the present invention does not require the use of extensive chemical processes in order to manufacture a substrate which has superior electrical characteristics, and can be more easily and accurately manufactured at a lower cost than prior art devices.
The present invention is closely related to issued and co-pending patents assigned to the assignee of the present invention, including U.S. Pat. No. 4,980,810, issued Dec. 25, 1990 and pertaining to "VHF DC-DC Power Supply Operating at Frequencies Greater than 50 Mhz"; U.S. Pat. No. 5,055,966, issued Oct. 8, 1991, on "Via Capacitors within Multi-layer, Three-Dimensional Structures/Substrates,"; and U.S. Pat. No. 5,164,699, issued Nov. 17, 1992, on "Via Resistors within Multi-layer, Three Dimensional Structures/Substrates." The related co-pending applications include Ser. No. 07/951,072, filed Sep. 24, 1992 on "Magnetic Vias within Multi-layer, Three Dimensional Structures/Substrates;" Ser. No. 07/951,504 filed Sep. 24, 1992 on "Field Control and Stability Enhancement in Multilayer 3-Dimensional Structures;" Ser. No. 07/923,409, filed Jul. 31, 1992, on "Low-Temperature-Cofired-Ceramic (LTCC) Tape Structures Including CoFired Ferromagnetic Elements, Drop-In Components and Multi-Layer Transformer;" and Ser. No. 07/951,473, filed Sep. 24, 1992 on "Dielectric Vias within Multi-layer Three Dimensional Structures/Substrates."